The technology of producing nanofibers has drawn special attention due to the unique properties nanofibers have compared to fibers with greater diameters made from the same materials. By decreasing the diameter of the fibers to a nanoscale, it is possible to increase significantly the surface volume with an improvement in the thermal and sound insulation. Furthermore, there is an increase in the liquid retention capacity, and changes occur in the texture and appearance.
The nanofibers can be comprised of various polymers, of synthetic or natural origin, and can be used for medical purposes, such as supports for tissues, controlled release of medicaments, and as curatives for skin regeneration. Important applications for micro and nanofibers have also been identified in non-medicinal products, such as air filters, protective clothing, sensors, electronics and matrices to immobilize catalysts, military applications and in cleaning utensils.
The majority of nanofibers are produced by melt spinning, electrospinning, or by hot air jets at high speed.
Melt spinning technology involves directing threads of the cast polymer to reduce the diameter of the fiber and induce the orientation of the polymeric chains. One of the limitations of melt spinning is that it is restricted to viscoelastic materials, which can withstand the efforts developed during the process. The diameter of the fibers made by this process is normally greater than 2 μm.
A variation of melt spinning for producing nanofibers is the islands-in-the-sea process, in which various individual matrices of a polymeric component are produced inside a single biggest thread of a second polymeric component. The bicomponent fibers are degraded at the same time using specialized equipment. A variation of this process that merely requires twin-screw extrusion equipment uses two immiscible polymers. The main limitation of this technique is the need for solvents to remove the sea component and the limited number of polymeric materials which can be treated in this manner.
A technique conventionally used for producing polymeric nanofibers is electrospinning (“electrospinning”). Electrospinning consists of the application of electrostatic and drag forces in the polymeric solution for forming nanofibers. The process includes an electrode connected to a positive (or negative) high voltage power supply inserted in the polymeric solution contained in a capillary tube. Initially, the solution is kept by its surface tension in the form of a drop at the end of the capillary. With the increase in the electrical voltage, the surface of the drop extends to form a cone (Taylor cone). When the electrostatic forces overcome the surface tension, a jet laden with the solution at the end of the cone is ejected. During the trajectory of the jet, the solvent evaporates and the polymer solidifies, forming a micro and/or nanofibrilar web that is deposited in an earthed metallic collector. Variables may influence the obtention of nanofibers through this process, such as the polymer/solvent concentration, electrical voltage applied in the solution, addition of salt in the solution, power stream (outflow of the solution from the capillary) and working distance (between the end of the capillary and the collector). The technique of electrospinning produces nanofibers with diameters in the range of 40 nm to 2 μm. Although electrospinning is considered the technique with the greatest potential for large scale production, the low efficiency in the production of fibers is still considered its greatest limitation. In the same way, the solvents compatible with electrospinning are limited by its dielectric constant. The process of electrospinning was patented in 1902, by J. F. Cooley (U.S. application Ser. No. 692,631) and W. J. Morton (U.S. application Ser. No. 705,691). Further developments for manufacturing textile yarns were achieved by Anton Formhals as of 1934 (U.S. Pat. Nos. 1,975,504 and 2,349,950).
Solution spinning is one of the oldest methods for producing nanofibers. This process includes wet spinning and dry spinning. In both methods, the viscous polymeric solution passes through fine holes disposed sequentially and the solvent is subsequently removed for producing the fibers, which are subsequently stretched to decrease their diameter and to confer orientation in order to increase their resistance. In dry spinning, the polymeric solution is pushed through a spinneret inside a heated column called spinning tower, in which the polymeric solution is solidified by evaporating the solvent. In wet spinning, a spinneret is placed in a chemical bath in which the polymer is precipitated by dilution or chemical reaction to form the fibers.
Another conventionally used technique for producing polymeric nanofibers is melt blowing. Melt blowing is a process for producing fibers directly from polymers, through the high speed of a gas jet or another suitable force to mitigate the filaments. The process can be controlled for producing fibers with diameters varying from 1 to 50 μm. Carl Freudenberg filed a patent application describing this process in 1965 (U.S. Pat. No. 3,379,811). The high-speed, hot gas process is also described in patents U.S. Pat. Nos. 3,276,944 and 3,650,866, among others. One of the limitations of high speed, hot air jet technology is that it is limited to the use of thermoplastic polymers.
Patent document WO2005033381 describes a method for electrospinning comprised by the steps of forcing the polymeric solution through a spinneret, in a first direction towards a collector situated at a distance from the first spinneret and, simultaneously, blowing the gas through the holes that are concentrically disposed around the spinneret. The method of this document uses electrospinning with gas jet, besides having an electrostatic force between the nozzle and the injector. In the present invention, the process does not use electrospinning or types of force or electrostatic force differential.
Patent documents CN101068956, U.S.2005067732, WO2006071977 and WO2006071976 use electrospinning for producing polymeric nanofibers. In the techniques described in these documents, the gas jet is an auxiliary component. In the present invention, the blowing is the fundamental component.
Patent document WO2005073442 describes an improved electrospinning technology for the continuous production of polymeric nanofibers from electrostatic spinning with the assistance of air injectors that direct and form the nanofibers. Besides using electrospinning, the method and the apparatus presented in this document use electrostatic forces.
Other patent documents that describe electrospinning technologies include, but are not limited to, WO2008062784, U.S.2008122142, WO2005042813, WO2005024101 and JP2008031624.
The contribution of the production of nanofibers to the growth of the fibers market depends on the development of new technologies, especially the development of large scale production processes.
The present invention describes an unprecedented process for gas jet spinning, comprising the use of elements of both electrospinning and high-speed hot gas jet technologies. The nanofibers produced in the present invention present the same diameters the fibers produced by electrospinning.